The Upper Rhine Plain, Rhine Rift Valley or Upper Rhine Graben (German: Oberrheinische Tiefebene, Oberrheinisches Tiefland or Oberrheingraben, French: Vallée du Rhin) is a major rift, about 350 km long and on average 50 km wide, between the cities of Frankfurt/Wiesbaden in the north and Basel in the south. Its southern section straddles the border between France and Germany. It forms part of the European Cenozoic Rift System, which extends across central Europe. The Upper Rhine Graben formed during the Oligocene as a response to the evolution of the Alps to the south and remains active to the present day. Today, the Rhine Rift Valley forms a downfaulted trough through which the River Rhine flows.